Flash memory with metal-insulator-metal tunneling program and erase

ABSTRACT

The flash memory cell comprises a sense transistor that has a pair of source/drain lines and a control gate. A coupling metal-insulator-metal capacitor is created between the control gate and a read wordline. A tunneling metal-insulator-metal capacitor is created between the control gate and a write/erase bit line. In one embodiment, the insulator is a metal oxide.

TECHNICAL FIELD OF THE INVENTION

This application is a Divisional of U.S. patent application Ser. No.10/881,662, filed Jun. 30, 2004 and titled, “FLASH MEMORY WITH METALINSULATOR-METAL TUNNELING PROGRAM AND ERASE,” which is commonly assignedand incorporated by reference in its entirety herein.

BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include personal computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code and system data such as a basic input/output system (BIOS)are typically stored in flash memory devices for use in personalcomputer systems.

FIG. 1 illustrates a cross-sectional view of a typical prior art memorycell having a metal-insulator-metal architecture. This cell has twosource/drain regions 101 and 102 that are doped into a silicon substrate100. A gate insulator layer 104 is formed over the channel between thesource/drain regions 101 and 102. A polysilicon layer 106 and metallayer 107 make up the floating gate 120 that stores the charge for thecell. A metal oxide acts as the intergate insulator 108 between thefloating gate 120 and the metal control gate 110.

In order to write data into the cell illustrated in FIG. 1, tunneling orhot electron injection would typically be used through the gate oxide104. Tunneling or hot electron injection can cause a degradation of thetransistor's characteristics, leading to read errors, after a number ofprogram and erase cycles.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora flash memory that reduces degradation due to program/erase cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a typical prior art flash memorycell.

FIG. 2 shows a cross-sectional view of one embodiment of a flash memorycell transistor array of the present invention.

FIG. 3 shows a schematic diagram of one embodiment of an electricalequivalent circuit of the cell of FIG. 2.

FIG. 4 shows a schematic diagram of one embodiment of an arrayconfiguration in accordance with the flash memory cell of FIG. 3.

FIG. 5 shows a graph of one embodiment of the programmed cellcharacteristics versus time in accordance with the flash memory cell ofFIG. 3.

FIG. 6 shows a block diagram of an electronic system of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof. The terms wafer or substrate used in thefollowing description include any base semiconductor structure. Both areto be understood as including silicon-on-sapphire (SOS) technology,silicon-on-insulator (SOI) technology, thin film transistor (TFT)technology, doped and undoped semiconductors, epitaxial layers of asilicon supported by a base semiconductor structure, as well as othersemiconductor structures well known to one skilled in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure, and termswafer or substrate include the underlying layers containing suchregions/junctions.

FIG. 2 illustrates a cross-sectional view of one embodiment of a flashmemory device structure of the present invention. Each device iscomprised of a sense field effect transistor (FET) 200 with a floatinggate and two metal-insulator-metal tunneling capacitors 201 and 202.

The sense FET 200 is comprised of a substrate 210 with two source/drainregions 211 and 212. One of the source/drain regions can act as the readbit/data line 211 while the other source/drain region 212 may be used asa ground connection. In one embodiment, the source/drain regions 211 and212 are n+ regions doped into a p-type silicon substrate 210. However,the present invention is not limited to any conductivity type or any onetype of substrate. The source/drain regions 211 and 212 are coupled by achannel region in which a channel forms during operation of thetransistor 200.

An oxide gate insulator 203 is formed on the substrate 210 over thechannel region between the source/drain regions 211 and 212. Apolysilicon layer 205 is deposited over the gate insulator 203. Afterdefinition and formation of this layer 203, a metal 204 is deposited ontop of the polysilicon 205 to act as the floating gate of the transistor200. In one embodiment, the metal layer 204 includes a barrier layer toprevent alloying of the metal 204 and polysilicon 205. The metal layer204 is annealed if necessary to provide smooth top and edge surfaceareas.

A metal oxide layer 206 is next formed on the metal layer 204 using lowtemperature oxidation of the metal, atomic layer deposition (ALD),evaporation, or some other oxidation technique. Examples of suchtechniques for purposes of illustration are discussed subsequently. Thismetal oxide insulator 206 forms the coupling 202 C_(C) and tunnelingcapacitor 201 C_(T) dielectrics.

Metal is deposited 207 and directionally etched and left on thesidewalls only. This forms the tunneling capacitor 201 C_(T). In oneembodiment, the tunneling capacitor 201 is on both sides of thetransistor 200. In an alternate embodiment, the metal is etched off toleave the metal 207 and tunneling capacitor on one side only. Forpurposes of clarity, the tunneling capacitor 201 of the embodiment ofFIG. 2 is shown on one side only.

The metal oxide 206 can then be selectively etched from the top of thefloating gate 220 and a new oxide formed on top of the floating gate 220and the write/erase bit/data line 207 before the wordline metal 208 isdeposited. This forms not only the coupling capacitor 202 C_(C) but alsoprovides electrical isolation between the metal write/erase bit/dataline 207 and the read data wordline 208.

In operation, the tunneling capacitor, C_(T), is a small area MIMcapacitor. It has a small capacitance that may be in the range of 5-10%of the coupling capacitor C_(C) that has a much larger area. A largevoltage applied to the write/erase bit/data line 207 appears mostlyacross this small tunneling capacitor 202. Because of the voltagedivision on the capacitors 201 and 202, the read wordline 208 may alsobe biased at the same time to increase the voltage difference across thetunneling capacitor 201.

For example, the write/erase bit/data line 207 might be biased at −9Vand the read wordline 208 at +3V. This would result in approximately 12Vacross the tunneling capacitor and an injection of electrons onto thefloating gate, thus leaving it with a negative potential ofapproximately −1V. A coincident address is required to write or erasedata onto the floating gate. Other cells that have only a bit/data lineor wordline bias will not have a large enough voltage difference acrossthe tunneling capacitor to cause the tunneling and/or change in datastored on the floating gate.

The cell of the present invention can be read by applying a readwordline voltage and determining the conductivity of the sensetransistor 200. In one embodiment, the wordline voltage might be 3V. Theabove-described voltages are for purposes of illustration only.Alternate embodiments may use other voltages.

If a positive charge is stored on the floating gate, the transistor 200will be highly conductive and is considered erased. If a negative chargeis stored on the floating gate, the transistor will not conduct and isconsidered programmed.

The embodiments of the flash memory cell of the present invention can beused with a differential sense amplifier. It provides excellentdiscrimination in stored charges states and data. The write and eraseoperations can also be interrupted and the sense transistor used tomonitor the progress of these operations. This enables multiple data bitstorage on a single cell.

In forming the metal oxide layer 206, the oxide growth rate and limitingthickness increases with oxidation temperature and oxygen pressure. Theoxidation kinetics of a metal may, in some cases, depend on thecrystallographic orientations of the very small grains of metal thatcomprise the metal films of the present invention. If such effects aresignificant, the metal deposition process can be modified in order toincrease its preferred orientation and subsequent oxide thickness andtunneling uniformity. To this end, use can be made of the fact thatmetal films strongly prefer to grow during their depositions havingtheir lowest free energy planes parallel to the film surface. Thispreference varies with the crystal structure of the metal. Metalorientation effects, if present, would be larger when only a limitedfraction of the metal will be oxidized and unimportant when most or allof the metal is oxidized.

Using a Pb/PbO/Pb structure, the Lead Oxide (PbO) barrier may becontrollably grown on deposited lead films using either thermaloxidation or RF sputter etching in an oxygen plasma. One processingsequence using such a thermal oxidation process includes starting with aclean polysilicon substrate and depositing a clean lead film on theoxide gate insulator at about 25° C. to 75° C. in a clean vacuum system.In one embodiment, the base pressure is approximately 10−8 Torr orlower. The Pb film will have a thickness within 1-2 Å of its targetvalue.

In one embodiment, lead and other metal films are deposited by aphysical sputtering process. The sputtering process offers the abilityto produce smoother films by increasing the re-sputtering-to-depositionratio since re-sputtering preferentially reduces geometric high pointsof the film.

A low temperature oxidation process is then used to grow an oxide filmof self-limited thickness. In one embodiment, oxygen gas is introducedat the desired pressure in order to oxidize the lead in situ without anintervening exposure to ambient air. For a fixed oxygen pressure andtemperature, the PbO thickness increases with log(time). Its thicknesscan be controlled via time or other parameters to within 0.10 Å asdetermined via in situ ellipsometric or ex situ measurements ofJosephson tunneling currents. This control over tunnel current is due tothe excellent control over PbO thickness that can be achieved by lowtemperature oxidation.

For example, increasing the oxidation time from 100 to 1,000 minutes atan oxygen pressure of 750 Torr at 25° C. only raises the PbO thicknessby 3 Å (e.g., from about 21 Å to 24 Å). Accordingly, controlling theoxidation time to within 1 out of a nominal 100 minute total oxidationtime provides a thickness that is within 0.1 Å of 21 Å. The PbO has ahighly stoichiometric composition throughout its thickness as evidencedfrom ellipsometry and the fact that the tunnel barrier heights areidentical for Pb/PbO structures.

Next, the system is re-evacuated and the top lead electrode isdeposited. This produces a tunnel structure having virtually identicaltunnel barriers at both Pb/O interfaces. The temperature used tosubsequently deposit the polysilicon control gate is not critical. ThePbO is stable to over 500° C. and thus introduces no temperatureconstraints on subsequent processes.

In another embodiment, Al/Al₂O₃ structures can be formed where the oxideis grown by low temperature oxidation in molecular or plasma oxygen.Capacitance and tunnel measurements indicate that the Al₂O₃ thicknessincreases with the log (oxidation time). This is similar to that foundfor Pb/PbO as well as other oxide/metal systems.

Additionally, tunnel currents for an Al₂O₃ tunnel barrier areasymmetrical with somewhat larger currents flowing when electrons areinjected from the Al/Al₂O₃ interface that is developed during oxidegrowth. This asymmetry is due to a minor change in the composition ofthe growing oxide. There is a small concentration of excess metal in theAl₂O₃, the concentration of which diminishes as the oxide is grownthicker. The excess Al ions produce a space charge that lowers thetunnel barrier at the inner interface. The oxide composition at theouter Al/Al₂O₃ contact is much more stoichiometric and thus has a highertunnel barrier. In spite of this minor complication, Al/Al₂O₃ tunnelingbarriers can be formed that produce predictable and highly controllabletunnel currents that can be ejected from either electrode. The magnitudeof the currents is still primarily dominated by Al₂O₃ thickness that canbe controlled via the oxidation parametrics.

In one embodiment of the present invention, Al₂O₃ metal oxidedielectrics can be formed by first thermally oxidizing the aluminum. Inother embodiments, the aluminum is plasma oxidized or other oxidationmethods can be used. Since the melting point of aluminum is much higherthan lead, the formation of the Al/Al₂O₃ structures are typicallysimpler than that used for the above-described Pb/PbO junctions.

In the Al₂O₃ metal dielectric process of the present invention, thealuminum is sputter deposited on an oxide or other insulator at atemperature in the range of approximately 25° C. to 150° C. Due tothermodynamic forces, the micro-crystals of the face centered cubic(f.c.c.) aluminum will have a strong and desirable preferredorientation.

The aluminum is then oxidized in situ in molecular oxygen usingtemperature, pressure, and time to obtain the desired Al₂O₃ thickness.As with the lead oxide, the thickness of the aluminum increases with log(time) and can be controlled via time at a fixed oxygen pressure andtemperature to within 0.10 Å when averaged over a large number ofaluminum grains that are present under the counter-electrode. Thethickness of the Al₂O₃ can be easily changed from about 15 Å to 35 Å byusing appropriate oxidation parametrics. The oxide will be amorphous andremain so until temperatures in excess of 400° C. are reached. Theinitiation of re-crystallization and grain growth can be suppressed, ifdesired, by the addition of small amounts of glass forming elements(e.g., Si) without altering the growth kinetics or barrier heightssignificantly.

The system is then re-evacuated and a layer of aluminum is depositedover the oxidized Al₂O₃ layer. Finally, the polysilicon control gatelayer is formed, using conventional processes that are well known in theart, on the layer of aluminum.

In the processes of the present invention, control over the propertiesof the various transition metal oxides is improved from the prior artdue to the limited thicknesses (approximately 10 Å to 100 Å) of metalthat precludes the formation of significant quantities of unwantedsub-oxide films. This is due to thermodynamic forces driving the oxidecompositions to their most stable oxidized state. In one embodiment, theduplex oxide layers are still crystallized. Such treatments can be doneby RTP and will be shorter than those used on MOCVD and sputterdeposited oxides since the stoichiometry and purity of the lowtemperature oxides need not be adjusted at high temperatures.

The above-described processes for low temperature oxidation of variousmetals are for purposes of illustration only. The present invention isnot limited to any one process for low temperature oxidation.

Single layers of Ta₂O₅, TiO₂, ZrO₂, Nb₂O₅ and similar transition metaloxides can be formed by low temperature oxidation of transition metalfilms in molecular and plasma oxygen. Examples of such operations arediscussed subsequently. These metal oxide layers can also be formed byALD, chemical vapor deposition (CVD), and RF sputtering.

These metals oxidize via logarithmic kinetics to reach thicknesses of afew angstroms to tens of thousands of angstroms in a temperature rangeof 100° C. to 300° C. Excellent oxide barriers for Josephson tunneldevices can be formed by RF sputter etching these metals in an oxygenplasma.

Lower temperature oxidation approaches of the present invention differfrom Metal-Organic Chemical Vapor Deposition (MOCVD) processes that areused to produce transition metal oxides. The MOCVD films typicallyrequire high temperature oxidation treatments to remove carbonimpurities, improve oxide stoichiometry, and produce re-crystallization.Such high temperature treatments might also cause unwanted interactionsbetween the oxide and the underlying silicon and, thus, necessitate theintroduction of interfacial barrier layers.

The embodiments of the present invention might also employ lowtemperature oxidation and short thermal treatments in an inert ambientatmosphere at 700° C. in order to form a range of perovskite oxide filmsfrom parent alloy films. The dielectric constants of crystallizedperovskite oxides can be very large (i.e., 100 to 1000). The transitionmetal layers can be either pure metals or alloys and the transitionmetals have similar metallurgy to their oxides. In contrast, the parentalloy films that can be converted to a perovskite oxide are typicallycomprised of metals having widely different chemical reactivities withoxygen and other common gasses.

If an alloy is to be completely oxidized, then thin film barriers suchas Pd, Pt, or their conductive oxides should be added between thesilicon and the parent metal film to serve as electrical contact layers,diffusion barriers, and oxidation stops. If the perovskite parent alloyfilm is only partially oxidized and covered with a second layer of theparent alloy, then the barrier heights will represent that developedduring oxide growth at the parent perovskite alloy/perovskite oxideinterface.

As is well known in the art, ALD is based on the sequential depositionof individual monolayers or fractions of a monolayer in awell-controlled manner. Gaseous precursors are introduced one at a timeto the substrate surface. Between the pulses the reactor is purged withan inert gas or evacuated.

In the first reaction step, the precursor is saturatively chemisorbed atthe substrate surface and during subsequent purging the precursor isremoved from the reactor. In the second step, another precursor isintroduced on the substrate and the desired films growth reaction takesplace. After that reaction, byproducts and the precursor excess arepurged from the reactor. When the precursor chemistry is favorable, oneALD cycle can be performed in less than one second in a properlydesigned flow-type reactor.

The most commonly used oxygen source materials for ALD are water,hydrogen peroxide, and ozone. Alcohols, oxygen and nitrous oxide havealso been used. Of these, oxygen reacts very poorly at temperaturesbelow 600° C. but the other oxygen sources are highly reactive with mostof the metal compounds listed above.

Source materials for the above-listed metals include: zirconiumtetrachloride (ZrCl₄) for the Zr film, titanium tetraisopropoxide(Ti(OCH(CH₃)₂)₄) for the Ti film, trimethyl aluminum (Al(CH₃)₃) for theAl film. Alternate embodiments use other source materials.

In another embodiment of the memory transistor of the present invention,the metal floating gate and metal oxide insulator layers can befabricated using evaporation techniques. Various evaporation techniquesare subsequently described.

Very thin films of TiO₂ can be fabricated with electron-gun evaporationfrom a high purity TiO₂ slug (e.g., 99.9999%) in a vacuum evaporator inthe presence of an ion beam. In one embodiment, an electron gun iscentrally located toward the bottom of the chamber. A heat reflector anda heater surround the substrate holder. Under the substrate holder is anozonizer ring with many small holes directed to the wafer for uniformdistribution of ozone that is needed to compensate for the loss ofoxygen in the evaporated TiO₂ film. An ion gun with a fairly largediameter (3-4 in. in diameter) is located above the electron gun andargon gas is used to generate Ar ions to bombard the substrate surfaceuniformly during the film deposition to compact the growing TiO₂ film.

A two step process in fabricating a high-purity ZrO₂ film avoids thedamage to the silicon surface by Ar ion bombardment. A thin Zr film isdeposited by simple thermal evaporation. In one embodiment, this isaccomplished by electron beam evaporation using an ultra-high purity Zrmetal slug (e.g., 99.9999%) at a low substrate temperature (e.g.,150°-200° C.). Since there is no plasma and ion bombardment of thesubstrate, the original atomically smooth surface of the siliconsubstrate is maintained. The second step is the oxidation to form thedesired ZrO₂.

The nitridation of the ZrO₂ samples comes after the low-temperatureoxygen radical generated in high-density Krypton plasma. The next stepis the nitridation of the samples at temperatures >700° C. in a rapidthermal annealing setup. Typical heating time of several minutes may benecessary, depending on the sample geometry.

A CrTiO₃ film can be created by first depositing a CoTi alloy film bythermal evaporation. The second step is the low temperature oxidation ofthe CoTi film at 400° C. Electron beam deposition of the CoTi layerminimizes the effect of contamination during deposition. The CoTi filmsprepared with an electron gun possess the highest purity due to ahigh-purity starting material. The purity of zone-refined startingmetals can be as high as 99.9999% and higher purity can be obtained indeposited films because of further purification during evaporation.

FIG. 3 shows a schematic diagram of one embodiment of an electricalequivalent circuit of the cell of FIG. 2. This diagram shows theconnections between the two capacitors 201 and 202 and the transistor200.

FIG. 4 illustrates an electrical equivalent circuit of the flash memorydevice of FIG. 2 showing memory array connections. The illustratedembodiment is a NOR architecture memory device. The present invention,however, is not limited to NOR memory arrays. Alternate embodiments mayuse NAND architecture arrays or other types of flash memoryarchitectures.

The circuit is comprised of the two MIM capacitors 201 and 202 and thetransistor 200 as illustrated in FIG. 3. The coupling capacitor C_(C)202 is coupled to the wordline 208 of the array. The tunneling capacitorC_(T) 201 is coupled to the write/erase bit/data line 207 of the array.One of the transistor's source/drain regions 211 is coupled to thearray's read bit/data line 211. The remaining source/drain region 212 iscoupled to ground. The array of FIG. 4 is in a NOR configuration.However, the embodiments of the present invention can also be arrangedin the NAND configuration or some other flash configuration.

FIG. 5 shows a graph of one embodiment of the programmed cellcharacteristics versus time in accordance with the flash memory cell ofFIG. 3. This graph shows that, even when a charge leaks off of thecapacitor in one of the possible charge states, a differential senseamplifier is still capable of reading valid data from the cell.

FIG. 6 illustrates a functional block diagram of a memory device 600that can incorporate the flash memory cells of the present invention.The memory device 600 is coupled to a processor 610. The processor 610may be a microprocessor or some other type of controlling circuitry. Thememory device 600 and the processor 610 form part of an electronicsystem 620. The memory device 600 has been simplified to focus onfeatures of the memory that are helpful in understanding the presentinvention.

The memory device includes an array of flash memory cells 630 that canbe the flash memory cells of the present invention that employ MIMtunneling. The memory array 630 is arranged in banks of rows andcolumns. The control gates of each row of memory cells is coupled with awordline while the drain and source connections of the memory cells arecoupled to bitlines. As is well known in the art, the connection of thecells to the bitlines depends on whether the array is a NANDarchitecture or a NOR architecture.

An address buffer circuit 640 is provided to latch address signalsprovided on address input connections A0-Ax 642. Address signals arereceived and decoded by a row decoder 644 and a column decoder 646 toaccess the memory array 630. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depends on the density and architecture of thememory array 630. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 600 reads data in the memory array 630 by sensingvoltage or current changes in the memory array columns usingsense/buffer circuitry 650. The sense/buffer circuitry, in oneembodiment, is coupled to read and latch a row of data from the memoryarray 630. Data input and output buffer circuitry 660 is included forbi-directional data communication over a plurality of data connections662 with the controller 610. Write circuitry 655 is provided to writedata to the memory array.

Control circuitry 670 decodes signals provided on control connections672 from the processor 610. These signals are used to control theoperations on the memory array 630, including data read, data write(program), and erase operations. The control circuitry 670 may be astate machine, a sequencer, or some other type of controller.

The flash memory device illustrated in FIG. 6 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of flashmemories are known to those skilled in the art.

CONCLUSION

In summary, the flash memory cell embodiments of the present inventionuse CMOS technology to produce a memory cell that does not usesemiconductor-oxide-polysilicon tunneling or channel hot electroninjection over transistor gate insulators as done in the prior art. Thepresent embodiments use two metal-insulator-metal (MIM) capacitors andone transistor. One MIM capacitor is used as a tunneling capacitor forboth program and erase operations while the second MIM capacitor is acoupling capacitor between the control gate and the wordline. Thepresent embodiments use only MIM tunneling.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A method for fabricating a flash memory cell, the method comprising:forming a pair of source/drain regions in a substrate material; formingan oxide gate insulator layer over the substrate and substantiallybetween the pair of source/drain regions; forming a floating gate layerover the gate insulator; forming a metal control gate layer over thefloating gate layer; forming a first metal oxide layer over the metalcontrol gate layer; depositing a metal layer that is directionallyetched to leave a metal-insulator-metal tunneling capacitor on asidewall of the cell; selectively etching the first metal oxide layer toform an isolation layer between the tunneling capacitor and the metallayer; forming a second metal oxide layer over the metal control gatelayer; and forming a metal wordline over the second metal oxide layer.2. The method of claim 1 wherein the metal oxide layer is formed by lowtemperature oxidation.
 3. The method of claim 1 and further includingannealing the metal control gate layer to provide smooth top and edgesurface areas.
 4. The method of claim 1 and further including forming abarrier layer prior to forming the metal control gate layer over thefloating gate layer.
 5. The method of claim 1 wherein forming the pairof source/drain regions includes doping the substrate to produce n+source/drain regions.
 6. The method of claim 1 wherein the metal controlgate, second metal oxide layer, and metal wordline form a couplingmetal-insulator-metal coupling capacitor.
 7. The method of claim 6wherein the coupling capacitor is comprised of a Pb/PbO/Pb structure andthe PbO is formed by thermal oxidation.
 8. The method of claim 6 whereinthe coupling capacitor is comprised of an Al/Al₂O₃/Al structure and theAl₂O₃ is formed by low temperature oxidation.
 9. The method of claim 1wherein the second metal oxide layer is comprised of one of: Ta₂O₅,TiO₂, ZrO₂, or Nb₂O₅.
 10. The method of claim 1 wherein the metalcontrol gate layer is comprised of one of yttrium, barium, or copper.11. The method of claim 10 wherein the second metal oxide layer isformed by thermally oxidizing the metal control gate layer to form aperovskite oxide film.
 12. The method of claim 1 wherein the secondmetal oxide layer is formed by one of: thermal oxidation, atomic layerdeposition, or evaporation.
 13. A method for fabricating a memory cell,the method comprising: forming a pair of source/drain regions in asubstrate material; forming a gate insulator layer on the substrate andsubstantially between the pair of source/drain regions; forming a chargestorage layer on the gate insulator; forming a metal control gate layeron the charge storage layer; forming a first metal oxide layer on themetal control gate layer; forming a metal layer that is directionallyetched to leave a metal-insulator-metal tunneling capacitor on asidewall of the cell; forming, from the first metal oxide layer, anisolation layer between the tunneling capacitor and the metal layer;forming a second metal oxide layer on the metal control gate layer; andforming a metal wordline on the second metal oxide layer.
 14. The methodof claim 13 wherein an oxide growth rate of the first and second metaloxide layers increases with oxidation temperature and oxygen pressure.15. The method of claim 13 wherein a limiting thickness of the first andsecond metal oxide layers increases with oxidation temperature andoxygen pressure.
 16. The method of claim 13 wherein the metal layer isdeposited using a sputtering process.
 17. The method of claim 13 andfurther including annealing the metal layer.
 18. A method forfabricating a memory cell, the method comprising: forming a pair ofsource/drain regions in a substrate material; forming a gate insulatorlayer on the substrate and substantially between the pair ofsource/drain regions; forming a floating gate layer on the gateinsulator; forming a metal-insulator-metal tunneling capacitor coupledto the charge storage layer; forming a metal-insulator-metal couplingcapacitor coupled to the charge storage layer; and forming a metalwordline on the coupling capacitor.
 19. The method of claim 18 whereinthe charge storage layer is a metal layer.
 20. The method of claim 19wherein the metal floating gate layer is one part of each of thetunneling and coupling capacitors.